The present invention relates to an improved multibeam electron gun for a cathode-ray tube and particularly to an electron gun having a modular beam-forming region (BFR) assembly comprising a plurality of cathode assemblies, a control grid (G1) electrode and a screen grid (G2) electrode. The electrodes have aligned apertures and are attached to a common ceramic support member. The screen grid electrode is positioned relative to the control grid electrode by support means which also accurately locates the BFR assembly relative to a main focusing lens of the electron gun.
U.S. Pat. No. 4,298,818 issued to McCandless on Nov. 3, 1981, discloses an electron gun having a modular beam-forming region (BFR) assembly similar to that of the present invention in that it also comprises a plurality of cathode assemblies and at least two successive electrodes including a control grid (G1) electrode and a screen grid (G2) electrode. Unlike the present invention, the successive electrodes of the patented beam-forming region are individually attached directly to metallized patterns on the surface of a common ceramic support member. The longitudinal spacing between the G1 and G2 electrodes is determined by the flange heights of the electrodes. A support bracket is embedded into the glass support rods of the electron gun in spaced relation to a main focusing lens. The screen grid electrode is welded to the support bracket to secure the modular BFR assembly in spaced relation to the main focusing lens. A drawback of the patented electron gun is that irregularities on the surface of the support ceramic or variations in the heights of the flange portions of the control grid or screen grid electrodes will cause variations in the longitudinal spacing between the successive electrodes. Proper operation of a multibeam cathode-ray tube utilizing such an electron gun requires that the spacing and alignment between the successive electrodes of the BFR assembly be accurately maintained. Apertures that are misaligned by as little as 0.0127 mm (0.5 mils) can cause distorted beam shapes and degrade the performance of the tube.
U.S. Pat. No. 4,500,808 issued to McCandless on Feb. 19, 1985, describes an improved electron gun similar to that of U.S. Pat. No. 4,298,818, except that the screen grid (G2) electrode of the modular beam-forming region (BFR) assembly comprises a composite structure including a metal support plate and three individual apertured plates. The metal support plate is brazed directly to a metallized pattern on one surface of a ceramic support member in spaced relation to a control grid (G1) electrode which is also brazed directly to a separate metallized pattern on the same surface of the ceramic support member. The metal support plate has a window therein opposite each of the apertures in the control grid electrode. The individual apertured plates are brazed to the metal support plate and close the windows therein. Each of the apertured plates has a single electron beam-defining aperture therein which is separately aligned with one of the apertures in the control grid (G1) electrode. This structure provides more accurate alignment of the G1 and G2 electrode apertures than previous structures; however, the longitudinal spacing between the G1 and G2 electrodes continues to depend on the flatness of the surface of the ceramic member and the flange heights of the G1 and G2 electrodes. Additionally, the longitudinal spacing between the screen grid (G2) electrode and the main focusing lens depends upon the thickness and flatness of the individual apertured plates brazed to the metal support plate of the G2 electrode.
An improved modular BFR assembly for an electron gun is described in U.S. patent application No. 769,978 filed by A. K. Wright on Aug. 27, 1985 entitled, "MULTlBEAM ELECTRON GUN HAVING MEANS FOR POSITIONING A SCREEN GRID ELECTRODE", assigned to the assignee of the present application. The electron gun of that application includes a modular BFR assembly and a main focus lens, both of which are affixed to a pair of insulative support rods. The BFR assembly includes a plurality of cathode assemblies, a control grid (G1) electrode and a screen grid (G2) electrode. The main focus lens includes a first focusing (G3) electrode and a second focusing (G4) electrode. The cathode assemblies and the G1 and G2 electrodes are individually held in position from a common ceramic member. A transition member having a flat first part and a second part electrically isolated from the first part is attached to a metallized pattern formed on one surface of the ceramic member. The second part of the transition member has a flat portion brazed to the metallized pattern and two upright portions that are substantially perpendicular to the flat portion and parallel to each other. The G1 electrode is attached to the first part of the transition member, and the G2 electrode is disposed between and attached to the upright portions of the second part of the transition member by means of a plurality of L-shaped support members. The longitudinal spacing between the G1 and G2 electrodes is set by means of a removable spacer. Each L-shaped support member has one end welded to the surface of the G2 electrode adjacent to the G3 electrode, and the other end welded to the upright support portions of the transition member. The upright support portions of the transition member permit a greater range in positioning the G2 electrode longitudinally in spaced relation to the G1 electrode than was available heretofore when each electrode was a precision formed part. The L-shaped supports allow the G2 electrode to be narrower than the width between the upright portions of the transition member so that the G2 electrode can be laterally positioned to align the electron beam-forming apertures with the corresponding apertures in the G1 electrode. The BFR assembly of patent application reduces the precision with which the G1 electrode and the surface of the ceramic member must be made, since the upright portions of the transition member provide a longitudinal tolerance not available in the prior electron guns described heretofore. A drawback of structure, however, is that since the BFR assembly is attached to the glass support rods by metal bead support members affixed at one end to the glass support rods and at the other end to the flat portion of the second part of the transition member, the spacing between the G2 and G3 electrodes in indirectly established with relation to the flat portion of the transition member. Thus, if the height of the G1 electrode or the flatness of the surface of the ceramic support member were to vary beyond the optimum range, corresponding variations in the location of the G2 electrode to maintain the G1 to G2 longitudinal spacing would result in an inverse variation in the G2 to G3 longitudinal spacing. The ends of the bead support members attached to the transition flange of the BFR assembly can be bent to provide the required G2 to G3 electrode spacing; however, such an expedient can cause cracking of the glass support rods or a subsequent change in G2-G3 electrode spacing as a result of the restorative force in the metal bead support members. An alternative is to provide electron guns having the bead support members attached to the glass support rods with a range of spacings between the ends of the bead support members and the G3 electrode to compensate for variations in the location of the G2 electrode. This is not practical in a high volume operation.